The present invention relates to apparatus for classifying the constituents of a tobacco-containing stream which is transported by a gaseous carrier medium. More particularly, the invention relates to improvements in apparatus for segregating relatively small particles of tobacco and/or other material from relatively large tobacco particles prior to introduction of larger particles into a further processing machine, e.g., into a shredding machine for tobacco leaves or into the distributor of a cigarette maker wherein the shreds are converted into a continuous rod-like tobacco filler. The particles which are to be separated from tobacco leaves or shreds may include sand, fragments of rock, short tobacco, tobacco dust and the like.
Many tobacco processing machines are coupled to each other by pneumatic conveyor systems. For example, shredded tobacco is often transported from one or more shredding machines to the next processing machine or machines in the form of a stream which is conveyed by a gaseous carrier medium, normally air. Upon arrival at the next processing station, the particles of the stream are separated from the carrier medium to be introduced into a hopper, magazine or another receptacle of the next processing machine. In many instances, the means for separating gaseous carrier medium from the particles of the stream comprises a rotary suction wheel which aspirates the carrier medium but prevents the entry of large solid particles into the path along which the carrier medium is withdrawn to be discharged into the atmosphere. It is desirable to separate the unwanted components of the stream simultaneously with segregation of gaseous carrier medium from the useful constitutents of the stream (i.e., from tobacco which is to be admitted into the next processing machine).
The situation is similar when the pneumatic conveyor system includes one or more pipes or tubes wherein the gaseous carrier medium circulates along an endless path to entrain particles of tobacco and other particles which are admitted into a first portion of the path and to be segregated from the particles on arrival of the particles into a second portion of the path. Such closed pneumatic conveyor systems are utilized when the gaseous carrier medium must be conditioned (e.g., by heating, cooling, raising the moisture content to a fixed level or reducing the moisture content to a predetermined level) because continuous conditioning of fresh supplies of a gaseous carrier medium would entail the consumption of excessive amounts of energy. The particles of tobacco are treated (e.g., dried, heated, cooled or moisturized) during transport along a portion of the endless path. As a rule, the gaseous carrier medium is cleaned (e.g., by passing through one or more filters) on its way from the second to the first portion of the endless path.
U.S. Pat. No. 3,116,238 to van Etten discloses a classifying apparatus wherein the outlet of the pneumatic conveyor discharges successive increments of the tobacco stream in a carrier medium into an arcuate channel which surrounds one side of an air-withdrawing wheel. The channel has a concave wall which is adjacent to the wheel and is perforated to permit the passage of dust. The particles of tobacco are intercepted and descend into an air lock. A drawback of such apparatus is that the perforations of the wall and the passages of the wheel are likely to be clogged with tobacco particles because the path of tobacco along such wall and along the wheel toward the air lock is relatively long. Moreover, the classifying action is not entirely satisfactory because the wall portion which faces the outlet of the pneumatic conveyor is not perforated.
Austrian Pat. No. 165,765 discloses an apparatus wherein the periphery of a rotary air withdrawing device is provided with a sieve which permits smaller particles, especially dust, to escape with the carrier medium. The particles of tobacco impinge upon a solid baffle and descend into an air lock. The sieve is cleaned by directing air from a portion of the interior of the air withdrawing device into the compartment which accumulates tobacco particles before the particles descend into the range of the air lock. A drawback of the patented apparatus is that substantial quantities of dust and other small particulate material escape with the carrier medium; therefore, such medium cannot be recirculated to the inlet of the pneumatic conveyor or it must be subjected to a very thorough and complex cleaning action. Moreover, a high percentage of dust and other small particulate material is likely to leave the apparatus with the acceptable material.
All presently known classifying apparatus which employ pneumatic conveyors exhibit the drawback that the classifying action is unsatisfactory, especially when the conveyor is to transport tobacco leaves whose specific weight is low. Such leaves cannot be readily separated from the carrier medium because their kinetic energy in the medium (normally air) is too low. Therefore, the leaves settle at the upstream side of a perforated wall or sieve and clog the orifices for the passage of air and/or tobacco dust. Such clogging is even more likely to occur when the path along which the leaves travel while in contact with the perforated wall is relatively long.